The fabrication of thermocouples using tubular metal sheaths is known in the art. Solomon in U.S. Pat. No. 3,973,997, Wagner in U.S. Pat. No. 3,980,504 and Nanigian in U.S. Pat. No. 4,732,619 teach a thermocouple formed between a tubular metal sheathing and an internal thermocouple wire. The tubular metal sheathing and the internal wire are electrically joined at one end to form a thermocouple junction where the metal sheathing is one of the thermocouple elements and the internal wire is the other. Alternatively, as taught by Gill in U.S. Pat. No. 4,512,827, two thermocouples wires, such as thermal couple wires 2 and 4 are encased in a common metal sheath, as shown in FIG. 1. The thermocouple wires 2 and 4 are insulated from each other and from the metal sheath by a powdered insulating material 8.
In this type of thermocouple, the powdered insulating material solidifies during the drawing process to reduce the diameter of the thermocouple assembly, thereby making it difficult to remove the material from between the thermocouple wires and the inner walls of the metal sheathing in order to form a thermocouple junction. In addition, the thermocouple wires must be selected from thermocouple alloys to compensate for changes in their calibration that occur during manufacture. The procedure for doing this is one of trial and error. It is only after processing that the thermo-electromotive force (EMF) of the resultant thermocouple assembly is determined to be suitable for standard or special limits of error. There is little that can be done after the sheathed thermocouple cable is completed.
In the alternative, Kleinle in U.S. Pat. No. 3,449,174 teaches a coaxial thermoelement in which the metal sheathing is made from the same metal or alloy as the coaxial thermocouple wire, while Black et al in U.S. Pat. No. 3,463,674 discloses a thermocouple in which a thermocouple junction is formed by two thermocouple wires joined end-to-end. The thermocouple junction and the thermocouple wires are covered by an insulating layer and enclosed by a common metal sheath. A second metal sheath is added in the vicinity of the thermocouple junction and the second metal sheath is reduced by drawing or swaging to a diameter substantially equal to the diameter of the first metal sheath.